Yale Researcher Challenges Traditional Role of Predators

"Revolutionary." That is the word Oswald Schmitz, Oastler Professor of Population and Community Ecology at the Yale School of Forestry & Environmental Studies, uses to describe the conclusions of research into how predators influence ecosystems. Pre-revolutionary logic: the soils -- and plants' interaction with soils -- determine the types of plants that grow. This in turn determines the types of herbivores present, thereby indirectly determining the population of carnivores. So effectively the ranges of wolves or cougars depend on the soil type. Reminds me of a joke...A group of TreeHuggers wants to join the eat organic, eat local movement so they decide to grow their own chickens. Unfortunately, all the chickens die.

Undeterred, the TreeHuggers buy a second lot of chicks and try again. This time, they plant the chicks with their heads above the ground. Unfortunately, the chickens all die again.

Abashed, the TreeHuggers write to the local agricultural extension, describing the first and second attempts at raising their chickens. The Ag, somewhat underfunded in the current political climate, replies: "Please send soil samples."

Unfortunately, mankind's ability to understand all the factors that influence natural ecosystems, and to use this knowledge to manage nature conservation, is not far removed from the talents of the comical subjects of this old joke. Professor Schmitz' research throws a whole new light onto the subject.

Schmitz studied two types of spiders in natural habitats isolated by cages. The jumping spider (Phidippus rimator) is the wolf of spiders, prowling about in search of tasty grasshopper prey. The nursery web spider (Pisaurina mira), on the other hand, is like a cougar: waiting patiently to ambush grasshoppers that wander too near.

The grasshoppers dine on various plants in their ecosystem. However, when being hunted by jumping spiders, the 'hoppers hide out in the goldenrod. Pinned down by the roving enemy, the grasshoppers must survive by eating the very goldenrod which shelters them. This represses the growth of the dominating goldenrod, allowing more species diversity to arise in the plant population. But ultimately, the soil pays the price as the nitrogen-fixing goldenrod is edged out by non-fixing plants.

In the tests with nursery web spiders, the ecosystem developed differently. Grasshoppers roamed more widely, feeding on a variety of plants. The goldenrod thrives, and the soil maintains a nitrogen richness.

Oswald Schmitz sums it up:

"What's really cool here is that different spiders have different hunting modes, and it's those modes that cause grasshoppers to behave differently, which then carries down the chains of the community structure of the plants," said Schmitz. "So it's a top-down view and, in that sense, it's revolutionary because it's a paradigm change in ecosystem ecology. Plants, ecosystem ecologists say, have an indirect effect on carnivores. My research shows that carnivores have an indirect effect on plants."